The present invention relates in general to continuously adjustable hydraulic-mechanical power transmissions having branched power outputs, and in particular to a transmission of the type which includes an input shaft cooperating with two clutches, a reversing shaft, a planetary gear set having three shafts of which one is selectively engageable by means of clutches either directly or via the reversing shaft with the input shaft, and a hydraulic transmission including a variable displacement hydraulic unit and a fixed displacement hydraulic unit, the second shaft of the planetary gear set being a secondary drive shaft, and the third shaft of the planetary gear set being directly connected to the fixed displacement hydraulic unit.
From German Pat. No. 2 415 002, a transmission is known which permits the same rotational speed at its output both in forward gear and in reverse gear. This known transmission consists of an adjustable hydraulic unit, of a fixed hydraulic unit, of a planetary gear set and of a reversing gear which is shiftable by means of a jaw clutch into forward, reverse or neutral position.
The adjustable hydraulic unit, the forward gear stage of the reversing gear, as well as the reversing stage of the reversing gear, are continuously in working connection with the input shaft of the transmission. The output shaft of the reversing gear is in engagement with the first shaft of the planetary gear set, the fixed hydraulic unit engages the second shaft, and the output shaft of the transmission engages the third shaft of the planetary gear set.
At full displacement of the adjustable hydraulic unit, the output shaft of the transmission stands still. If the displacement of the adjustable hydraulic unit is adjusted to lower values, the output shaft starts rotating. In continuing to reduce the displacement, a correspondingly increased rotational speed at the output is attained. When the adjustment of the adjustable hydraulic unit crosses zero, then the direction of rotation of the fixed hydraulic unit is reversed, resulting in a further increase of rotational speed at the output. The maximum rotational speed at the output of the transmission is obtained when full adjustment in the opposite direction is completed.
A substantial disadvantage of this known construction is in the mode of shifting from the forward to the reverse gear. Due to the fact that the toothed wheels of the reversing gear rotate in opposite directions for the forward and reverse motion, and a flawless engagement with the output shaft of the reversing gear is possible only upon synchronization, it is necessary that, upon disengaging a gear stage for one direction, the adjustable hydraulic unit be readjusted from one extreme position to the opposite extreme position before the other gear stage may be synchronously engaged. To meet this requirement, an expensive coupling between the clutch for the direction of movement and the adjusting means for the hydraulic unit is necessary. Moreover, such prior-art transmission does not enable a fast shift from one direction to the other, inasmuch as the readjustment of the hydraulic unit from one extreme angular position of its adjuster to the opposite extreme angular position takes always a certain amount of time. An additional disadvantage from the viewpoint of control technology is also to be seen in the fact that the two ranges of speed of movement start at opposite maximum angular positions of the adjustable hydraulic unit.